In rotating electrical machines, e.g. generators, subslots, open at opposite ends of a rotor, typically underlie the field windings of the rotor. Radial cooling slots extend through the windings in communication with the subslots. A cooling medium is supplied and flows from opposite ends of the rotor through the subslots and the radial cooling ducts of the field windings and exits into the gap between the rotor and the stator. Thus the cooling medium, for example air, is in direct heat exchange relation with the coils of the rotor field windings.
This type of cooling ventilation system has proven satisfactory particularly for small machines having a limited length, e.g. on the order of less than 200 inches. For larger machines, for example rotors in excess of 200 inches, the long rotors limit the flow of the cooling medium through the subslots. That is, the cumulative cross sectional area of the subslots is considerably less than the cumulative cross sectional area of the radial ducts. This causes a large pressure drop at the entrance locations to the radial cooling ducts and insufficient flow of the cooling medium into the middle or intermediate portions of the rotor.
Various arrangements have been proposed and constructed to provide additional cooling to the intermediate portions of the rotors in these longer rotating machines. For example, scoops at the inlet holes have been used to force the cooling medium into the windings by impact between the cooling medium and the rotating rotor. Diagonal cooling medium flow ducts have also been utilized where the cooling medium flows inwardly to the bottom of the coils through holes slightly axially offset in successive turns and is allowed to return through other similarly axially offset holes to the rotor surface and gap. Reverse flow ventilation systems have also been utilized such as disclosed in U.S. Pat. No. 5,652,469. However, there remains a need for providing improved cooling and ventilation particularly in intermediate regions of the rotor in longer electrical machines.